Here is a first selection for beginners in Japanese, around JLPT N5 to N4 levels. The following web mangas are made for children, in a picture book style. Later in the month I'll give another selection for a bit more mature audience.
Author : ひろこっこ
Website : くまいぬつうしん
Chapters : 135 (ongoing)
Kumainu and Chibikuma's adventures in the forest. That's about all.
Here it's about as close to a picture book as you can get: every page is composed of a big picture followed by a few typewritten lines. This is what makes Kumainu Tsûshin ideal for a beginner. Since the text is short you can read a little, stop and read again without having the impression of being in front of an insurmountable wall. Also the text isn't part of the images so as soon as there's a word you don't understand, you can copy-paste it in an online dictionary.